Among the 12 turtles equipped with IMASEN, nine individuals returned to the nesting beach. All had lost their magnet and/or Hall sensor and two had also lost their logger. Of the seven returned loggers, three had no recorded data. Consequently, only four turtles provided data for recorded periods varying from 7·3 h (no. 4) to 56·1 h (no. 2) (Table 1).
overall diving behaviour
The 1009 dives recorded from the four turtles were shallow (11·8 ± 6·3 m) and short (6·6 ± 2·6 min, Table 1), with 50% and 75% of overall dives shallower than 10·7 m and 16·7 m and shorter than 6·2 min and 8·2 min, respectively (Fig. 3). Within a dive, the time spent at the bottom lasted on average 2·6 ± 2·1 min (i.e. 38·9 ± 23·2% of the total dive duration). Wiggles occurred in 65·3% of the dives, with a mean of 2·4 ± 1·0 wiggles per dive. The mean hourly diving effort was 46·0 ± 12·1 min spent diving per hour.
Figure 3. Frequency distribution of (a) dive depth and (b) dive duration for all recorded dives (n = 1009 dives, in black), dives without beak movement events (n = 667 non-b-m dives, in grey) and dives associated with beak movement (n = 342 b-m dives, in white) in four leatherback turtles during one internesting interval in French Guiana in 2006.
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V-, U- and W-shaped dives represented 37·7 ± 15·4%, 54·3 ± 17·3%, and 8·0 ± 4·0% of overall dives, respectively (mean ± SD of the four turtles, Table 2). U-shaped dives were shallower than V- and W-shaped dives (Table 2). W-shaped dives were longer than U- and V-shaped dives (Table 2) due to their longer bottom time.
Table 2. Summary of dive parameters and beak movement (b-m) occurrence in relation to dive shape (U, V and W) in four leatherback turtles during one internesting interval in French Guiana in 2006. Differences in means and proportions were tested statistically using χ2, analysis of variance (anova) followed by a post-hoc Tukey test or Kruskal–Wallis test followed by a post-hoc Bonferroni test. Different letters (a, b) indicate significant (P < 0·05) differences among groups. Values are expressed as mean ± standard deviation
|Dive type||All dives||U-shaped||V-shaped||W-shaped||Test, P-value|
|Frequency (%)||–||54·3 ± 17·3||37·7 ± 15·4|| 8·0 ± 4·0||χ2, χ22 = 180·8, P < 0·001|
|Dive depth (m)||11·8 ± 6·3||11·1 ± 6·6a||12·5 ± 5·9b||13·1 ± 5·7b||anova, F2,1007 = 7·53, P < 0·01|
|Dive duration (min)|| 6·6 ± 2·6|| 6·5 ± 2·5a|| 6·3 ± 2·3a|| 9·4 ± 2·8b||anova, F2,1007 = 60·1, P < 0·001|
|% with b-m events||32·7 ± 5·0||27·8 ± 8·3a||38·2 ± 7·8a, b||64·7 ± 25·4b||Kruskal–Wallis, H2,12 = 7·54, P < 0·05|
|No. of b-m events per b-m dives|| 1·8 ± 1·4|| 1·9 ± 0·5a|| 1·5 ± 0·5a|| 1·6 ± 0·4a||Kruskal–Wallis, H2,12 = 1·76, P = 0·415|
dives associated with beak movement events
Among the 1009 recorded dives, 342 dives were associated with at least one beak movement event (i.e. 32·7 ± 5·0%, range: 26·2–36·8%, Fig. 2). Those dives associated with beak movements (hereafter called ‘b-m’ dives) were mainly shallow (14·0 ± 6·6 m) and short (7·5 ± 2·8 min), but were also deeper (t-test, t342,667 = 7·88, P < 0·001) and longer (t342,667 = 7·77, P < 0·001) than the remaining 667 non-b-m dives (dive depth: 10·7 ± 5·9 m; dive duration: 6·1 ± 2·4 min, Fig. 3). Among the 342 b-m dives, 50% and 75% were shallower than 13·4 and 19·4 m, and shorter than 7·4 min and 9·5 min, respectively (Fig. 3). The turtles performed between 2·3 and 2·6 b-m dives per hour (Table 1). The b-m dives were isolated throughout time rather than distributed in successive bouts (sensu Gentry & Kooyman 1986).
The proportion of b-m dives among overall dives increased significantly with dive depth and dive duration when either considering each turtle individually (Spearman's rank correlation, P < 0·05 in all cases) or when considering all turtles together (Spearman's rank correlation for the grand mean of dive depth: RS = 0·913, n = 4 turtles, P = 0·011, Fig. 4a, grand mean of dive duration: RS = 0·866, n = 4 turtles, P < 0·001, Fig. 4b).
Figure 4. Proportion of dives associated with beak movement events among all recorded dives in relation to (a) dive depth and (b) dive duration in four leatherback turtles during their internesting interval in French Guiana in 2006. Individual relations were calculated with all recorded dives for each turtle (turtles 1, 2, 3 and 4 in open circles, open squares, open triangles and open diamonds, respectively) and for pooled data (mean ± standard deviation, bold open circles, n = 4 individuals).
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Regarding dive shapes, b-m dives were mainly U-shaped and V-shaped (44·7 ± 13·9% and 42·2 ± 10·9%, respectively, for the four turtles). W-shaped dives represented only 13·1 ± 3·8% of the b-m dives. However, among all W-shaped dives, 64·7 ± 25·4% were associated with beak movements (i.e. 4·6% of the 1009 recorded dives) compared to 27·8 ± 8·3% of all U-shaped dives (i.e. 14·1% of the 1009 recorded dives) and 38·3 ± 7·8% of all V-shaped dives (i.e. 15·2% of the 1009 recorded dives, Table 2).
occurrence of beak movement events during the dive
Beak movements were recorded both at the surface and during dives. In both situations, three categories of beak movement events were defined: (a) isolated beak movements (lasting between 1·5 and 4 s); (b) group of two to five successive beak movements (lasting between 3 and 20 s in total); and (c) series of more than five successive beak movements (lasting between 10 s and 5 min in total; Fig. 2). Less than 1% of the events recorded at the surface were isolated beak movements. The category of a beak movement event recorded during a surface time was not linked to the occurrence or the category of beak movement events in the previous or following dive.
Among the b-m dives, beak movement events occurred on average 1·8 ± 1·4 times per dive (range: 1–11, Tables 1 andFig. 2). The mean number of beak movement events per dive was not significantly different on comparison of the three types of dive profiles (Table 2). Beak movement occurrence was mainly isolated (55·0 ± 12·2% of the events) and grouped (41·0 ± 9·1% of the events), rather than in series (4·0 ± 3·8% of the events; Tables 1 and Fig. 2). Thus, 96·0 ± 4·0% of beak movement events during a dive lasted between 1·5 and 20 s.
There was one single isolated and one single grouped beak movement event in 36·8 ± 9·7%, and in 25·5 ± 8·4% of the b-m dives, respectively. There were several isolated and several grouped beak movements in 12·6 ± 4·5%, and in 11·6 ± 4·9% of the b-m dives, respectively. In 5·1 ± 3·9% of the b-m dives, isolated beak movements preceded groups of beak movements, while it was the contrary for 3·0 ± 2·6% of the b-m dives. This pattern was similar when considering U-shaped and V-shaped b-m dives separately. However, for W-shaped b-m dives, a single group of beak movements occurred in 50·3 ± 33·8% of the cases, and groups of beak movements preceded isolated beak movements in 10·4 ± 10·8%. In addition, W-shaped dives presented significantly more grouped beak movement events than other types of beak movement events, whereas U-shaped and V-shaped dives presented significantly more isolated and grouped beak movement events than series of events (Table 3).
Table 3. Proportions of beak movement (b-m) occurrence and types (isolated, grouped, in series) in relation to dive shape (U, V and W) in four leatherback turtles during one internesting interval in French Guiana in 2006. Differences in means and proportions were tested statistically using a Kruskal–Wallis test followed by a post-hoc Bonferroni test (n = 4 turtles). Different letters (a, b) indicate significant (P < 0·05) differences among groups
|Dive type||% of b-m events in descent phase||% of b-m events in bottom phase||% of b-m events in ascent phase||Test P-value||% isolated b-m events||% grouped b-m events||% series of b-m events||Test P-value|
|U-shaped||13·7 ± 13·0a||71·6 ± 12·6b||14·7 ± 7·8a||H2,12 = 7·4, P < 0·05||57·3 ± 18·4a||38·7 ± 15·2a||4·0 ± 3·3b||H2,12 = 8·0, P < 0·05|
|V-shaped||29·6 ± 17·6a||55·8 ± 23·4a||14·7 ± 8·0a||H2,12 = 5·5, P = 0·06||58·9 ± 15·7a||37·1 ± 13·8a||4·0 ± 4·6b||H2,12 = 9·0, P < 0·05|
|W-shaped|| 8·7 ± 13·8a||84·8 ± 20·3b|| 6·5 ± 7·0a||H2,12 = 7·4, P < 0·05||24·9 ± 18·7a||72·1 ± 21·2b||3·0 ± 3·5a||H2,12 = 8·3, P < 0·05|
No significant increase was seen in the mean number of beak movement events per b-m dive with dive depth or dive duration, when considering either each turtle individually [analysis of variance (anova), P > 0·05 in all cases except turtle no. 2, which showed an increase with dive depth, F3,124 = 3·14, n = 127 dives, P = 0·03] or all turtles together (Kruskal–Wallis, dive depth: H5,18 = 4·14, n = 4 turtles, P = 0·53; dive duration: H13,43 = 18·3, n = 4 turtles, P = 0·14).
During U-shaped and W-shaped b-m dives, beak movement events occurred mainly during the bottom phase (71·6 ± 12·6% and 84·8 ± 20·3%, respectively, Table 3, Figs 2 and 5) whatever their category (isolated, group or series). During V-shaped dives, beak movements occurred similarly during the descent (29·6 ± 17·6%), the bottom (55·8 ± 23·4%) or the ascent phase of the dive (14·7 ± 8·0%, Table 3, Figs 2 and 5) except for isolated beak movements, which occurred mainly during both the descent and the bottom phase of the V-shaped dives (38·8 ± 16·6% and 44·4 ± 22·8%, respectively).
Figure 5. Frequency of beak movement events recorded during each phase of the dive (descent, bottom and ascent) for the three types of dives (U, V and W) associated with beak movements (b-m dives) in four leatherback turtles during one internesting interval in French Guiana in 2006. Kruskal–Wallis followed by a post-hoc Bonferroni test (*P < 0·05; NS: P > 0·05).
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There was no significant relationship between the proportion of time spent at the bottom of the dive and the number of beak movement events per dive, whatever their category (Pearson's correlation, P > 0·05). The majority of beak movement events (62·7%) corresponded to a change in swimming direction, i.e. either a wiggle during the bottom phase (in 28·0 ± 8·0% of the recorded cases, n = 4 turtles), or the beginning (11·9 ± 8·7%) and the end (7·9 ± 5·1%) of the bottom phase, or the peak of the V-shaped dives (11·4 ± 1·4%), or a ‘dive step’ (3·5 ± 5·1%) (Fig. 2). The percentage of beak movement events associated with a wiggle was not significantly different among the three categories of beak movement events (H2,11 = 3·3, n = 4 turtles, P = 0·19). On average, 39·5 ± 21·4% and 36·1 ± 12·3% of groups and series of beak movements, respectively, were associated with a wiggle and 23·9 ± 2·5% of isolated beak movements.